Metal removal from water, such as mine drainage and industrial wastewater is important for many reasons, including reducing or avoiding drinking water contamination and other environmental contamination, and to comply with government regulations. Mine drainage is a byproduct of hard rock mining operations that expose sulfide containing minerals to water and oxygen, inducing sulfide mineral oxidation. Both active and abandoned hard rock mines are sources of millions of gallons of runoff each year. The runoff is generally characterized by very low pH, elevated concentrations of dissolved iron and sulfate, and depending on the local geology, a suite of other dissolved metal cations and complexes. Example metal cations found in many of these waters include zinc, copper, mercury, lead, gold, silver, cadmium, uranium, chromium, among other metals. Industrial wastewater includes that which is generated by leather tanning operations, textile manufacturing, electronic “chip” manufacturing, metal plating facilities, nuclear fuel and nuclear weapons processing, and electric power generation (both nuclear and coal), to name only a few examples.
Many of these metal-containing waters are hazardous to humans, animals, and plants, and therefore have been mandated for treatment by government regulations. There are several known approaches for removing metals from water, although none of these approaches work well for a broad spectrum of waste streams.
Among the remediation strategies that have been used to treat or otherwise recover metals from water are controlled precipitation, membrane separation processes and immobilization on ion exchange resins. Controlled precipitation is generally accomplished by adding sufficient amounts of base (e.g., carbonate addition) to a metal-containing water in order to shift chemical conditions to a point where metals have lower solubility and thus precipitate as solids. One of the disadvantages of this approach is that the alkalinity that needs to be added to the water to drive reliable precipitation of metals are well in excess of natural levels, and the corresponding reagent masses and volumes used to adjust the alkalinity can be costly. In general, precipitation processes generate large amounts of metal-laden sludge that is difficult and costly to collect and transport from the treatment site for disposal.
Ion exchange resins have also been used to remove metals from solution. Ion exchange generally involves introducing a metal-containing water through a resin bed (often configured as a packed column), to immobilize metal ions using spheroid beads, which include an active resin or zeolite. Metals are exchanged on a charge equivalent basis for nonmetal species, which are liberated into solution as the metals are sequestered from solution. Disadvantages of this approach include the resin performance being sensitive to pH (needing to operate in a narrow pH range), the effluent containing other ions, and is relatively expensive to implement. Ion exchange processes are also sensitive to the presence of suspended particulate matter through a broad range of particle size distributions, such that pretreatments are often needed.
There are other approaches, which attempt to remove metals from water. While these processes become significantly less effective, or become ineffective as pH levels drop (less than about 7).
Still other approaches attempt to use metal-coordinating organic compounds, many of which themselves participate in acid/base reactions, to enhance metal immobilization on activated carbon. These attempts use metal binding agents, such as Ethylenediaminetetracetate (EDTA), porphyrin and porphyrin-containing compounds, citrate and citrate-containing compounds and dimercaprol. Use of metal binding agents such as these, share two difficulties. There is limited enhancement of the immobilization of metals, as compared with activated carbon used alone. And process efficiency drops markedly in response to dropping pH levels (i.e., these attempts fail to be significant at pH levels less than about 4.5).
The occupational exposures, limitations and costs of alkalinity remain a major challenge in terms of engineering advancements toward developing more robust and cost-effective treatment alternatives to remediate metal-contaminated waters.